The U.S. Food and Drug Administration will request Sarepta Therapeutics to voluntarily stop all shipments of its gene therapy, Elevidys, a source familiar with the matter told Reuters on Friday.

Shares of the company extended losses and were down nearly 35% in afternoon trading, after being halted briefly.

The FDA's request follows the death of two teenage boys who received Elevidys, a gene therapy approved in the United States to treat a muscle-wasting condition called Duchenne muscular dystrophy.

Both the boys were non-ambulatory patients, or those who could not walk independently, and in June, Sarepta had stopped shipments of the therapy to that patient group.

Sarepta did not immediately respond to a Reuters request for comment.

Earlier on Friday, the company disclosed the death of another patient who received one of its experimental gene therapies, deepening investor concern over use of its treatments.

A 51-year-old man enrolled in an early-stage trial of its gene therapy, SRP-9004, for the treatment of limb-girdle muscular dystrophy (LGMD) died from acute liver failure last month.

FDA Commissioner Marty Makary told Bloomberg News in an interview that he was "taking a hard look" at whether a gene therapy from Sarepta should remain on the market.